Piston-and-cylinder assemblies are used in a variety of fields, including combustion engines, hydraulics, and pneumatics. Typical assemblies include a piston slideably mounted within a cylinder to fill, pressurize, and evacuate substance in the cylinder. This can include air-fuel mixtures, liquids, and gasses.
FIG. 1 illustrates a typical assembly 20 in which a piston 22 is slideably received within a cylinder bore 24 formed in a block 26. A cylinder head 28 is attached to the block 26 to form a chamber 30 between the top surface 32 of the piston 22 and an inside surface 34 of the cylinder head. Because the piston 22 has a diameter slightly smaller than the diameter of the cylinder bore 24, a space 36 is created therebetween. While this reduces friction between the piston 22 and the wall 38 of the cylinder bore 24, it enables substance to pass from the chamber 30 to outside the cylinder bore 24. In other words, when the piston 22 slides upward in the cylinder 24 towards the cylinder head 28, compression of substance within the chamber 30 will force some of the substance into the space 36 between the piston 22 and the cylinder wall 38. To prevent this from happening, resilient rings are employed.
More particularly, the selection of piston rings for an engine or other combustion application is related to the particular type of engine. Because different engines have varying requirements, such as competitive racing engines, truck engines, sport engines, and engines designed for specific fuels such as diesel, aircraft, automobile, as well as alcohols and nitrous oxides; all may require specific differences in materials and design. Piston rings function to contain and maintain in the cylinder chamber 30 a combustion pressure, to prevent oil from getting into the combustion chamber 30, and to assist in the control of temperature in the engine.
As shown in FIG. 1, there are three rings comprising a top ring known as a compression ring 40, a second ring or secondary compression ring 42, and a third ring or oil control ring 44. The top ring 40 aids in sealing against loss of pressure during the combustion process. It is designed to maintain a high buildup of pressure as the piston arrives at the top of its stroke and when the combustible mixture is ignited, building up additional pressure to force the piston 22 downward. Several design criteria aid in the ability of the piston ring to maintain this pressure, including ring gap, material resiliency, and the size and spacing of the ring 40 with respect to the piston 22 and the cylinder wall 38.
The second compression ring 42 is similar to the first compression ring 40 in that it has a ring gap that allows gasses to further penetrate down the space 36 between the piston 22 and the cylinder wall 38. This passing of the hot gasses is known as blow-by, and can have detrimental affects on the engine. This includes contaminating the oil with carbon particles from the combustion process, raising the acidic level and heating up the oil and speeding up the oxidation process. This in turn allows the carbon particles to wear out all the parts that the oil is expected to lubricate. This ring also serves as an oil scraper to minimize oil above the second ring 42.
The oil ring 44 is designed to aid in lubrication of the other rings, pistons, the rod, wrist pins, and cylinder walls while preventing the oil from interfering with the combustion process. The oil ring 44 also assists in the thermal control of the piston 22 by aiding in passing oil to the inside of the piston for cooling as well as lubrication.
Referring next to FIG. 2, conventional ring terminology is illustrated. This includes the scuff band A, which is one or more raised bands of piston material used in some piston designs to reduce scuffing. The groove depth B is the distance between the back of the ring groove and the cylinder wall with the piston centered. The groove root diameter C represents the piston diameter measured at the back of the groove. This may vary between grooves on the same piston.
The land diameter D is the diameter of a given land. This can also vary by design from the top to the bottom of the piston. The land clearance E is the difference in diameter between the cylinder bore and the land diameter. “E” represents one-half of the total.
The skirt clearance F is the difference in diameter between the cylinder bore and the piston skirt diameter. In this case “F” represents one-half the total difference. The skirt groove G is a ring groove cut below the pin bore to carry an oil ring. The pin bore offset H is the distance the pin bore is offset from center and the groove spacer I is used on re-grooved pistons to return a ring groove to specifications or, in some performance applications, to facilitate the use of narrower ring sets than for which the grooves were originally designed.
FIG. 3 illustrates ring terms and measurements wherein the free gap A is the ring end clearance when the ring is uncompressed. The compressed gap B, also known as the ring gap, is the end gap measured when the ring is installed.
The radial wall thickness C is the distance between the inside and outside faces of the ring wall. The ring diameter D is the diameter of the ring measured with the ring installed on the piston, and the inside diameter E is measured with the ring installed on the piston. The ring sides F is the top and bottom surfaces of the ring. The ring face G is the part of the ring that is in contact with the cylinder wall. The side clearance H is the clearance between the ring groove and the ring side F.
The ring width I represents the thickness of the ring between the top and bottom faces F. Torsional twist J as shown in FIG. 3D is a result of an imbalance in the compression of the upper and lower sides F of the ring, causing the ring to twist when compressed. This torsional twist is accounted for when attempting to seal both the ring in the groove and the ring to the cylinder wall. Finally, the back clearance K is the distance between the inside diameter of the ring and the bottom of the ring groove when the ring is installed on the piston.
In the design of combustion engines, there are certain applications in which high power and torque in the 1000-2000 rpm range is desired. Because most engines are designed to operate at a higher rpm range, the low performance in the 1000-2000 rpm range is largely due to friction, and most of it caused by ring friction. Various designs have been proposed for reducing ring friction while maintaining performance levels. For example, U.S. Pat. No. 4,596,179, is directed to a reciprocating machine having a cylinder, a piston performing reciprocating movement within the cylinder and rollers mounted on each side portion of the piston in rolling contact with the cylinder. As shown in FIG. 1 of this patent, the rollers are mounted below the rings and are designed to maintain the piston in alignment with the cylinder and prevent friction resulting from side thrust and lateral oscillating movements of the connecting rod as exerted on the piston. The use of these rollers would be inappropriate for sealing the piston in the cylinder because their design permits large amounts of blow-by.
U.S. Pat. No. 4,442,759 describes a piston and cylinder in a hydraulic power booster having rollers mounted in a piston groove and maintained in radial contact with the cylinder wall by way of a leaf spring. These rollers are received in grooves in the cylinder wall to prevent torsional twisting of the piston due to forces exerted on a spindle coupled thereto. These rollers would be ineffective in preventing blow-by and sealing a combustion chamber.